In general, a disk device (also referred to as a disk drive) typified by a hard disk drive is provided with a magnetic head for recording data in or reproducing data from a magnetic disk medium (hereinafter simply referred to as a magnetic disk) which is a recording medium. As the magnetic head, a composite head is used in which a read head and a write head are separate and mounted on the same slider. The read head generally comprises a magnetoresistive (MR) element or a giant magnetoresistive (GMR) element, and performs a read operation (data reading operation). The write head generally comprises an inductive thin film head element, and performs write operation (data writing operation).
This kind of head is generally installed in a rotary actuator. The actuator is configured to be driven to rotate radially on a disk medium by the driving force of a voice coil motor (VCM) and thereby position the head at a target position (target track or target cylinder) on the disk medium.
Meanwhile, when the head is positioned on the disk medium by the rotary actuator, a gap distance (Grw) is present in a circumferential direction because the read head and the write head are separate. Moreover, since the read head and the write head are differently positioned radially on the disk medium due to the angle of rotation of the actuator, a so-called offset is generated.
Thus, the disk drive performs offset control for offset-based positional adjustments of the read head and the write head in order to position the magnetic head at the target position on the disk. The offset control is performed on the basis of an offset value that changes depending on the radial position on the disk. However, this offset value is a value that does not change over one revolution of the disk in the same track.
On the other hand, in the disk drive, a phenomenon known as a disk run-out is caused, for example, by an attachment error of a spindle motor or by an external shock or vibration. When the disk run-out is caused, a servo track deviation (servo track run-out) is generated from a turning circle locus on the rotation center of the disk. Therefore, accurate offset control of the magnetic head is not enabled by the offset value that does not change over one revolution of the disk in the same track.
In order to solve such a problem, a technique called dynamic offset control (DOC) is suggested wherein the offset value is changed on the basis of a disk run-out amount to perform offset control (e.g., Jpn. Pat. Appln. KOKAI Publication No. 2009-176403 (paragraph 0081)).
In a disk drive described in the patent publication, a servo pattern for offset measurement is reproduced in a particular zone of a non-servo region other than a servo region of the disk, thereby calculating an offset value that changes within one revolution of the disk. In a manufacturing process of the disk drive, when no disk run-out is caused after the first calculation of an offset value OF(s), the calculated offset value OF(s) is not updated and maintained as it is. On the other hand, when a positional error of the read head resulting from the DOC operation is beyond a threshold which is a reference value, it is determined that the disk run-out is caused. In this case, position information for offset measurement is again recorded, and a new offset value OF(s) is again calculated by the above series of calculation steps, thus updating the offset value stored in a memory.
However, this is not effective in the device described in Patent document when data recorded before the disk run-out is reproduced after the disk run-out in a disk drive which only performs the DOC during reproduction. When the DOC is only performed during reproduction, data is recorded in accordance with the disk run-out during recording. For the data recorded before the disk run-out, the read DOC has to be performed by use of an offset value suited to the disk run-out amount during recording. If the read DOC is performed by use of the offset value suited to the disk run-out amount after the disk run-out, a correction which is not suited to the disk run-out during recording is made. As a result, data access performance more deteriorates due to the DOC, and an error rate (BER) may rise.